A slide-fastener stringer using a continuous monofilament coupling element generally comprises a support tape to one edge of which is affixed a continuous coupling element or coil which interdigitates with an opposing coupling element or coil on the edge of another support tape upon movement of a slider along the stringer.
When the coupling element consists of a coil, e.g. of polyester synthetic-resin monofilament, each turn of the coil comprises a coupling member which is formed on one side with a coupling head receivable between the turns or coupling heads of the opposite coupling element.
Each turn may have a lower shank adapted to rest against the tape and an upper shank over which passes the stitching, e.g. double-lock stitch or chain stitching, which secures each coupling element to the support tape. The coupling heads may be deformed transversely to the length of the synthetic-resin monofilament, i.e. parallel to the axis of the coil so that projections of the coupling head reach behind the coupling heads of the opposing coupling element. Such coupling members are thus chained together by being formed on the synthetic-resin monofilament.
Since the coupling elements are formed by coiling a synthetic-resin monofilament, stresses arise during the coiling operation which generally must be relieved by a process which can be termed thermofixation. In this process the monofilament is heated, usually after coiling, to a temperature sufficient to cause internal relaxation and hence retention of the pitch of the coupling element, i.e. the interhead spacing, and relief of the internal stresses formed by the coiling process. This temperature may be as much as or slightly less than the softening point of the synthetic-resin monofilament.
In general, the thermofixation is purely a stress-relieving operation after the synthetic-resin monofilament has been coiled into its final shape, thereby fixing this shape. The thermofixing can be carried out after the monofilament has been coiled and while the turns of the coupling element lie in beds in a forming tool.
The heating can be effected by ultrasonic energy. It has not been possible, with these earlier systems, to provide the turns of the coupling element with thread-receiving recesses in which the threads by which the coupling element is attached to the tape can be positively positioned and retained so that the stitching threads do not project materially beyond the outlines of the coupling element and hence are subject to wear by the slider.
A recess of the type referred to should, therefore have a depth equal at least to the thickness of the stitching thread.
The recessing of the threads of the retaining seam in the turns of the coupling element has been found to be especially necessary when the slide fastener is used as a shoe closure along the inside of a boot or shoe and hence may be subjected to wear not only by the slider but in actual walking operations.
If one attempts to provide such thread-receiving recesses by embossing or the like, once the coupling element has been fabricated completely, it is found that one obtains not only the recesses but a practically uncontrolled alteration in the shape of the coupling member which results in variation in the depth to which each coupling member engages in the opposing coupling element but also a broadening of the synthetic-resin monofilament in the region of the recesses which alters the pitch (i.e. interhead spacing) and causes problems during the attachment of the coupling element to the tape by sewing.
When, however, attempts are made to avoid these problems by forming the recesses by removal of material, e.g. milling or cutting, the synthetic-resin monofilament is impermissibly weakened.